Older women are more likely to conceive twins because rising concentrations of a naturally occurring hormone over-stimulate their ovaries, according to a new study.

The discovery by a team of scientists from Amsterdam’s Vrije Universiteit, revealed in Thursday’s edition of the journal Human Reproduction, solves a riddle that has been perplexing the medical profession for years.

“The findings give credence to previous theories that the rising concentrations of follicle stimulating hormone (FSH) that occur as women age can cause some ovaries to go into overdrive, tripping them into a state where they have simultaneous double ovulation,” they said.

Identical twins come from a fertilized egg dividing to develop two babies. Non-identical twins occur when two eggs are fertilized at the same time.

One of the researchers, Cornelius Lambalk, said the discovery helps explain why there is a surge in the number of non-identical twins born to older women when their general state of fertility is declining due to age.

“This rise has not been entirely due to treatment for infertility,” he said.

“About half of the increase has been caused by the number of spontaneous multiple pregnancies probably due to the fact that women are delaying childbirth to a later age.”